AC drives and variable frequency drives are both terms used to refer to equipment designed to provide variable frequency power to an AC motor. Typically, components of an AC drive include a rectifier and an inverter. The rectifier operates to rectify constant frequency AC power to produce DC power across two lines, known as a DC link, which connect the rectifier and the inverter. The inverter operates to invert the DC power to produce variable frequency AC power that is used to power the AC motor.
The rectifier is commonly constructed using diodes or controllable solid-state semiconductor based switching devices such as silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate-commutated thyristors (GCTs), and/or insulated gate bipolar transistors (IGBTs). To rectify three phase AC power, typically six controllable switching devices are selectively controlled via pulse width modulation or the like.
To control a three-phase AC motor, the inverter also typically includes six controllable switching devices to link the DC power to the motor. By switching these inverter switches on and off, the three phases of the motor are linked to the positive and negative lines of the DC link in a controlled fashion such that variable frequency power is delivered to the motor.
When a motor driven by an AC drive is rapidly decelerated, power may be regenerated and directed back to the DC link. The regenerated energy is normally consumed by mechanical losses and by the motor, but excess energy can also be stored in a bus capacitor connected across the DC link. Many AC drives include means to monitor the voltage across the DC link, and when this voltage exceeds a predetermined value, a power resistor is used to dissipate the excess energy. Typically, this power resistor is in series with a switch across the DC link and the switch is closed when excess energy needs to be dissipated. The switch may be provided as a built-in so-called seventh transistor of the inverter or may be an added component. Similarly, the power resistor may be provided as a built-in component of the AC drive or may be an added component.